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△H rxn

Discussion in 'MCAT Study Question Q&A' started by EMT313, Sep 4, 2014.

  1. EMT313

    Jul 3, 2014
    Likes Received:
    When calculating △Hrnx how do we know if we have to do products - reactants, or if we need to do △H bonds broken - △H bonds formed (which would be reactants - products essentially). I am very confused on this and maybe my thinking is completely off. Can anyone clear this up?
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  3. justadream

    7+ Year Member

    Apr 29, 2011
    Likes Received:

    If you are given stuff relating to bond energies, use bonds broken - bonds formed (which is Reactants - Products).

    If you are doing normal enthalpy stuff not relating to bonds, then do products - reactants.

    Sorry if this is not the most scientific answer but that's just how I remember it. I'm pretty sure the instance I mentioned above is the only time you ever do Reactants-Products.

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